Proximity detection system for deep wells

Abstract

A method and apparatus for magnetic field measurements to determine the proximity of a nearby target incorporating electrically conductive material includes a drill string ( 54 ) having multiple drill pipe sections ( 56, 57, 58, 59 ) connected end-to-end, with at least one of the drill pipe sections ( 57 ) being electrically conductive and isolated to provide an electrode section. A nonmagnetic drill pipe section ( 84 ) is connected in the drill string below the electrode section ( 57 ), and a hydraulic motor ( 62 ) having a rotatable drill bit sub ( 70 ) carrying a magnetic field sensing instrument package ( 102 ) is connected to a lowermost end of the drill string. A power supply provides a time-varying current to the drill pipe electrode section ( 57 ) to produce a corresponding target current magnetic field to be detected at the drill bit instrument ( 10 ) 2 , and a communication instrument package ( 94 ) is locatable within the nonmagnetic drill pipe section ( 84 ) to receive magnetic field data from the magnetic field sensing instrument package ( 102 ) on the drill bit ( 70 ).

1. Apparatus for target proximity detection from a borehole being drilled, comprising:
 a drill string having multiple drill pipe sections connected end-to-end and carrying a drill bit; 
 at least one of said drill pipe sections being electrically conductive to provide a drill pipe electrode section; 
 at least one electrically insulating drill pipe sub electrically isolating said electrode section from adjacent drill pipe sections; 
 a power supply in electrical communication with said at least one drill pipe electrode section and energizable to inject a time-varying current into Earth formations surrounding said borehole; 
 a drill bit instrument at said drill bit for detecting magnetic fields produced by said injected current; and 
 communication electronics located in said drill string for establishing communication between said drill bit instrument and surface instrumentation for sending detected magnetic field data to said surface instrumentation. 
 
     
     
       2. The apparatus of  claim 1 , further including:
 a wireline locatable within said drill string, said wireline incorporating a wireline electrode connected to said power supply for electrical communication with said drill string electrode for injection of said time-varying current into Earth formations; and 
 wherein said wireline carries a part of said communication electronics for communication of detected magnetic field data to said surface instrumentation. 
 
     
     
       3. The apparatus of  claim 2 , wherein said drill string further includes:
 first and second electrically conductive drill pipe electrode sections spaced apart along the length of the drill string and electrically isolated from each other and from adjacent drill pipe sections by electrically insulating drill pipe subs; and 
 first and second electrodes on said wireline, said wireline electrodes being spaced to be located within and in electrical communication with respective first and second drill pipe electrode sections when said wireline is inserted in said drill string. 
 
     
     
       4. The apparatus of  claim 1 , wherein said drill bit instrument incorporates first magnetic field sensors for detecting vector components of time-varying magnetic fields characteristic of said injected current and second magnetic field sensors for detecting static magnetic field vectors. 
     
     
       5. The apparatus of  claim 4 , wherein said communication electronics includes drill bit telemetry in said drill bit instrument and wireline telemetry carried by said wireline. 
     
     
       6. The apparatus of  claim 1 , wherein said drill string further includes:
 first and second electrically conductive drill pipe electrode sections spaced apart along the length of the drill string and electrically isolated from each other and from adjacent drill pipe sections by electrically insulating drill pipe subs; and 
 wherein said power supply is located in said drill string and is electrically connected to said first and second drill pipe electrode sections. 
 
     
     
       7. The apparatus of  claim 6 , wherein said power supply is a battery. 
     
     
       8. The apparatus of  claim 1 , further including a directional drilling assembly connected at a distal end of said drill string between said drill pipe sections and said drill bit to drive said drill bit. 
     
     
       9. The apparatus of  claim 8 , further including:
 a wireline locatable within said drill string, said wireline carrying at least one wireline electrode for electrical connection to said at least one drill string electrode and carrying said communication electronics for location within a non-magnetic drill string section above said directional drilling assembly, said drill bit being located below said directional drilling assembly; 
 magnetic field sensors mounted in said drill bit instrument for detecting said varying magnetic fields; 
 an orientation package located in said drill bit instrument; 
 wherein said communication electronics includes a transmitter in said drill bit instrument for transmitting data from said magnetic field sensors and said orientation package to said surface instrumentation. 
 
     
     
       10. The apparatus of  claim 9 , wherein said communication electronics further includes a receiver carried by said wireline for receiving said data and retransmitting it to said surface instrumentation. 
     
     
       11. The apparatus of  claim 9 , wherein said drill bit instrument is mounted in a housing at said drill bit, the apparatus further including an AC amplifier mounted in said drill bit instrument for measuring electric fields between said directional drilling assembly and said housing to resolve sign ambiguity of detected magnetic fields. 
     
     
       12. Apparatus for deep well measurements in a borehole being drilled to determine the distance and direction from the borehole to a target, comprising:
 a drill string in said borehole, the drill string having multiple drill pipe sections connected end-to-end and carrying at a distal end a directional drilling assembly and a drill bit sub; 
 at least one of said drill pipe sections being electrically conductive to provide an electrode section; 
 at least one electrically insulating drill pipe sub electrically isolating said electrode section from adjacent drill pipe sections; 
 a non-magnetic drill string section between said drill bit sub and said at least one drill pipe electrode section; 
 a power supply connectable to said drill pipe electrode section to inject current into the earth surrounding said borehole and to produce a corresponding current flow in said target; 
 a drill bit instrument at said drill bit sub and including first magnetic field sensors for detecting vector components of time-varying magnetic fields produced by said current flow in said target; 
 communication electronics within said nonmagnetic drill pipe section; and 
 telemetry for transmitting data corresponding to said vector components from said drill bit instrument to said communication electronics. 
 
     
     
       13. The apparatus of  claim 12 , wherein said drill string includes two electrically conductive drill pipe electrode sections spaced apart along the length of the drill string and electrically isolated from each other and from adjacent drill pipe sections by electrically insulating drill pipe subs, and wherein said power supply is connected to both of said electrically conductive drill pipe sections. 
     
     
       14. The apparatus of  claim 13 , wherein said power supply is located at the Earth's surface and is connected to said drill pipe electrode sections by a wireline insertable into said drill string, the wireline incorporating a power cable and two wireline electrodes, each wireline electrode being locatable within a corresponding drill pipe electrode section and connected through said power cable to said power supply;
 wherein said wireline carries said communication electronics; and 
 wherein said wireline incorporates a data cable connected to said communication electronics for transmitting data to surface instrumentation. 
 
     
     
       15. The apparatus of  claim 13 , wherein said power supply is located in said drill string and is connected to said drill pipe electrode sections;
 the apparatus further including a measurement while drilling (MWD) instrument in said non-magnetic drill string section and connected to said communication electronics for transmitting data received by said communication electronics to instrumentation at the Earth's surface. 
 
     
     
       16. A method for deep well measurements in a borehole being drilled for determining the distance and direction from the borehole to a target, comprising:
 locating a drill string in said borehole, the drill string having multiple drill pipe sections connected end-to-end and carrying at a distal end a directional drilling assembly and a drill bit sub; 
 providing at least one electrically conductive drill pipe electrode section in said drill string; 
 electrically isolating said electrode section from adjacent drill pipe sections; 
 energizing said drill pipe electrode section to inject a time-variable current into the earth surrounding said borehole and to produce a corresponding time-variable current flow in said target; 
 detecting vector components of time-varying magnetic fields produced by current flow in said target at a drill bit instrument at said drill bit sub; 
 transmitting data corresponding to said time-varying magnetic field to a communication instrument package located within a nonmagnetic drill pipe section located above said drive motor; 
 transmitting said data from said communication instrument package to surface instrumentation; and 
 determining from said data the distance and direction from said drill bit sub to said target. 
 
     
     
       17. The method of  claim 16 , further including detecting vectors of the earth's magnetic field and gravity at said bit instrument to obtain Earth field perturbation data; and
 transmitting said Earth field perturbation data to said communication instrument package. 
 
     
     
       18. The method of  claim 16 , further including supplying said time-variable current to two spaced-apart drill stem electrodes to produce corresponding time-varying currents in the earth surrounding the borehole and in said target. 
     
     
       19. The method of  claim 18 , wherein supplying said time-variable current comprises supplying a transient pulsed current having a decay period to produce a corresponding decaying current having a decay period in said target, the decaying target current producing a corresponding characteristic decaying target magnetic field. 
     
     
       20. The method of  claim 19 , further including detecting vectors of said time-varying magnetic fields produced by current flow in said target during the decay period of said target current. 
     
     
       21. The method of  claim 20 , further including transmitting said vector data from said communication instrument package to a computer for determining the location of said target with respect to said drill bit sub. 
     
     
       22. The method of  claim 16 , further including resolving the sign ambiguity of detected magnetic fields.